The use of molecular sieves as catalysts in the alkylation of aromatic feedstocks is well known in the art. Two molecular sieve catalysts which may be employed in such alkylation processes are ZSM-5 and silicalite. ZSM-5 zeolites and methods for their preparation are disclosed in U.S. Pat. No. 3,702,886 to Argauer et al and U.S. Pat. No. 3,941,877 (Re 29,948) to Dwyer et al. Silicalite and its preparation are disclosed in U.S. Pat. No. 4,061,724 to Grose et al.
Both silicalite and ZSM-5 type catalysts can be characterized very generally in terms of mole oxide ratios as follows: EQU x M.sub.2 O:Al.sub.2 O.sub.3 :y SiO.sub.2
Wherein:
M is an alkali metal cation, normally sodium PA0 x is the mole ratio of alkali metal oxide to alumina, and PA0 y is the silica/alumina ratio.
For the traditional ZSM-5 zeolite, y ranges from about 5 to about 100 although in the case of highly siliceous ZSM-5 zeolites of the type disclosed in Dwyer, termed metal organosilicates, y can be substantially greater than 200. x is about 1, resulting in an Na.sub.2 O content in the molecular sieve structure which is normally above 1 wt. % although in the case of the organometal silicates of Dwyer, the Na.sub.2 O content can be substantially less. In the case of silicalite, y is normally greater than 200, typically about 300, although it may be substantially greater, and x is usually substantially less than 1. Expressed in terms of wt. % of the molecular sieve structure, the Na.sub.2 O content is usually less than that of ZSM-5 and normally less than 1 wt. % in any event.
It has been proposed in various prior art processes involving either ZSM-5 zeolites or silicalite to adjust the sodium content of the catalyst. Thus U.S. Pat. No. 4,441,991 to Dwyer et al directed to hydrodewaxing over a highly siliceous porous crystalline catalyst "related to" the zeolite ZSM-5, and having a silica/alumina ratio greater than 200, discloses that it is preferred to reduce the sodium or other alkali metal content of the as synthesized zeolite to a value of less than 50% of the original sodium content. By thus reducing the sodium content, the product as used will have a sodium concentration of less than 0.5 wt. % and usually less than 0.1. Dwyer goes on to state that the sodium content is preferably less than 0.03% and most preferably less than about 0.01%. While the Dwyer disclosure is principally directed to crystalline materials prepared in accordance with Dwyer Re 29,948, the patent also refers to silicalite as a catalyst "related to" the zeolite ZSM-5.
U.S. Pat. No. 4,309,276 to Miller discloses the use of silicalite in a hydrocarbon conversion process involving the production of olefins from a paraffinic feed. Miller discloses that the sodium content of the silicalite should be less than 0.1 wt.% and most preferably less than about 0.01 wt.%. Specifically disclosed are silicalites having sodium contents of 140 and 400 ppm, corresponding to 189 and 539 ppm, Na.sub.2 O, respectively.
As noted previously, both ZSM-5 type zeolites and silicalite are known in the art as catalysts for use in the alkylation of aromatic substrates. U.S. Pat. No. 4,387,260 to Watson et al discloses the alkylation of aromatic compounds such as benzene or toluene over silicalite in which a water cofeed is employed to prolong the cycle length of the process between catalyst regenerations. The water cofeed is supplied in an amount preferably within the range of 20,000-100,000 ppm based upon the aromatic substrate and more preferably within the range of 20,000-60,000 ppm. Silicalite is also effective in the production of ortho-suppressed and para-enhanced dialkyl aromatic compounds as disclosed in U.S. Pat. Nos. 4,489,214 (Butler et al) and 4,490,570 (Forward et al). It is also an effective catalyst in the production of alkyl aromatic compounds such as ethyl benzene where sulfur is present in the feedstock either as a contaminant in the ethylene stream or in the benzene stream, as disclosed in U.S. Pat. No. 4,587,371 to Forward et al.
The aforementioned patents do not address the subject of the sodium content of the silicalite catalysts. Where ZSM-5 type zeolites have been used in the alkylation of aromatic compounds it has been proposed to use ZSM-5 zeolites of moderately low sodium content. For example, U.S. Pat. No. 4,139,600 to Rollmann et al discloses the synthesis of low sodium ZSM-5 crystalline aluminosilicalites said to be useful in processes including the alkylation of aromatics with olefins. Exemplary materials given in the Rollman patent characterized as being "very low" in sodium had less than 0.4% sodium. In terms of the amount relative to aluminum, the Na.sub.2 O/Al.sub.2 O.sub.3 ratio of the ZSM-5 products was below 0.25. U.S. Pat. No. 4,104,319 to Kaeding discloses the ethylation of a monoalkyl benzene with two ZSM-5 catalysts. One of a moderately high silica/alumina ratio of 70/1, had a sodium content of 0.05 wt.%. The other, having an extremely high silica/alumina ratio of 1670 also had a substantially higher sodium content of 1.6 wt.%.